linux-kernel - Re: [RFC][PATCH 0/5] Signal scalability series

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Message-ID: <20111003131610.GA26823@redhat.com>
Date:	Mon, 3 Oct 2011 15:16:10 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	Tony Luck <tony.luck@...el.com>
Subject: Re: [RFC][PATCH 0/5] Signal scalability series

On 09/30, Matt Fleming wrote:
>
> On Fri, 2011-09-30 at 18:52 +0200, Oleg Nesterov wrote:
>
> > Hmm. Just out of curiosity, I blindly applied the whole series and poke
> > the _random_ function to look at, dequeue_signal(). And it looks wrong.
> >
> > 	spin_lock_irqsave(&current->signal->ctrl_lock, flags);
> > 	current->jobctl |= JOBCTL_STOP_DEQUEUED;
> >
> > This signal->ctrl_lock can't help. A sig_kernel_stop() should be
> > dequeued under the same lock, and we shouldn't release it unless we

s/unless/until/

> > set JOBCTL_STOP_DEQUEUED. Otherwise we race with SIGCONT.
>
> Hmm.. is that really a problem? Does the dequeue and setting
> JOBCTL_STOP_DEQUEUED actually need to be atomic?

It should be atomic wrt SIGCONT.

> Does it matter if we
> have SIGCONT on the signal queue when we set JOBCTL_STOP_DEQUEUED?

Why do we have? Usually SIGCONT is ignored. But this doesn't matter,
SIGCONT acts at the sending time.

If SIGCONT is sent - the process must not stop. Since we drop the lock
we can't guarantee this.

> > May be do_signal_stop() does something special? At first flance it doesn't.
> > But wait, it does while_each_thread() under ->ctrl_lock, why this is safe?
>
> Why is it not safe? What scenario are you thinking of where that isn't
> safe?

This series doesn't add ->ctrl_lock into copy_process/__unhash_process
or I misread the patches. This means we can't trust >thread_group list.

Even this is safe (say, we can rely on rcu), we can't calculate
->group_stop_count correctly. In particular, without ->siglock we can
race with exit_signals() which sets PF_EXITING. Note that PF_EXITING
check in task_set_jobctl_pending() is important.

Oleg.

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